Electrical components comprising resin cast inside a shell



p 1961 R. G. BLACK 3,001,004

ELECTRICAL COMPONENTS COMPRISING RESIN CAST INSIDE A SHELL Filed July25, 1959 1 A5 I I WITNESSES: Fig 3, lNVENTOR Richard G. Black StatesPatent O This invention relates, generally, to electrical compo nentsand, more particularly, to components which are' formed by casting aresin inside a shell of a material other than the resin.

Casting resins are frequently utilized for electrical insulation inapplications-which requirethe resin to be pouredinto a'shell whichserves as a mold during the curing process. The mold is sometimesremovedtthereafter for reuse, but in certain applications is allowed toremain with the casting. When more than an ounce or so of resin is usedfor such casting, the volumetric shrinkage during curing is of such amagnitude that separationusually. occurs between the body of resin andthe walls of the shell. This makes no difference when the only functionof the shell'is to serve as a mold or as mechanical. protection, butwhen a tight and continuous bond is-requiredbetween the' resin and thecontainer it creates a problem. It is particularly serious in caseswhere the shell is of metal which is insulated by the resin from aninternal conductor maintained at high electrical potential, as incertaininsulating bushings. Any voids between theresin and the metal undersuchcircumstances may give, rise to thephenomenon of internal corona,thereby causing radio interference.

An object of the invention is to prevent the formation Another object ofthe invention is to utilize a struca tural part of an electricalcomponent as a mold for easting the component.

Other objects of the invention will be explained fully hereinafter orwill be apparent to those skilled in the art.

In accordance with the invention, separation of a casting resin from ashell during curing is prevented by perforating or slotting the shell insuch a manner that the shell can shrink with the resin and remain intotal contact therewith.

For a better understanding of the nature and objects of the invention,reference may be had to the following detailed description, taken inconjunction with the accompanying drawing in which:

FIGURE 1 is a view, in elevation of an insulating bushing embodying theprincipal features of the invention;

FIG. 2 is a view, in elevation, of a grounding flange and metal sleeveutilized in the bushing shown in FIG. 1; and

FIG. 3 is an isometric view of a metal shell which may be utilized forcasting a component of an electrical system.

Referring to the drawing, and particularly to FIG. 1, the terminalbushing shown therein is of the type fully described in a copendingapplication of C. F. Sonnenberg, Serial No. 829,091, filed July 23,1959. The bushing comprises a cylindrical metal sleeve which is securedto a metal mounting flange 11, a conductor 12 which extendslongitudinally through the sleeve 10 and is spaced from the sleeve, athermoset resin body 13 and a porcelain casing '14. The sleeve 10 andmounting flange 11 are preferably composed of aluminum since aluminumhas substantially the same thermal eoeflieient of expansion as thethermoset resin material utilized in casting the body. If a resin havinga diflerent coefficient of expansion is utilized, the sleeve 10 shouldbe composed of a metal 3,001,004 Patented Sept. 19, 1961 havingsubstantially the same coefiicient of expansion as the resin body. Inall cases themetal shell should not have a coetficient of expansionmarkedly different from the resin utilized. As shown in FIGS. 1 and 2,the sleeve 10 is welded to the flange 11. However, the sleeve 10 and theflange llmay-be formed as one piece.

As described in the aforesaid copending application, a removable mold(not shown) is provided at the lower end of the sleeve 10. As alsodescribed in the copending application, the porcelain casing 14 may beutilized as part of the mold for-casting the resin body 13. In order toprevent voids from forming between the resin body 13 and the casing '14during the curing process, the casing is lined with a flexible andelastic material, such as, silicone rubber or a, neoprene base rubbercement.

The thermoset resin utilized for the body 13 should be one that has goodphysical strength, a relatively low shrinkage during curing, and hasgood adherence to metal and ceramic, such as porcelain and glass.Polyester resin compositions and any of the thermosetting casting resinscommonly utilized for electrical insulation may be utilized in thepresent structure. The resinous polymeric epoxides such as athermosetting resin comprising a glycidyl polyether of a polyhydricphenol having a l, 2 epoxy equivalency of between 1 and 2, have beenfound to meet the foregoing requirements and have been metal sleeve 10and the grounding flange 11 are provided to distribute the voltagestress over a greater surface area. Inthe present structure, the sleeve10 is utilized as part of the mold for casting the resin. i v

, As shown in FIGS. 1 and 2, the sleeve 10 is provided with a pluralityoflongitudinal slots 15 and 16 around These slots permit the adtheperiphery of the sleeve. hesion of the resin to the metal to pull thesegments of the metal shell inwardly during the curing process, therebypreventing shrinkage of the resin from forming voids between the metalshell and the resin. When the bushing is utilized for a relatively highvoltage service, any voids between the resin and the metal may give riseto the phenomenon of internal corona, causing radio interference. Whenthe metal sleeve is slotted, in the manner shown, the metal contractswith the resin during curing, thereby preventing the formation of voidsbetween resin and the metal.

When the slots are provided in the metal sleeve, a rubber sleeve orother temporary means, such as aluminum foil, may be provided around thesleeve 10 to retain the resin within the sleeve during the castingprocess. The temporary molds may be removed after the resin is cured.

An insulating bushing having a resin body cast inside a slotted metalsleeve was-tested for radio interference and the results compared withthe radio interference of a bushing in which an impregnated fibrousmaterial was utilized :as the insulation. The cast resin bushing causedmuch less radio interference at the higher voltages. Thus, the presentbushing is more suitable for high voltage service than bushings of aprior construction in which the insulation was an impregnated fibrousmaterial.

The metal shell shown in FIG. 3, and which is of a boxlike shape, may beutilized as a mold for containing a casting resin for encasingelectrical components of an electrical system. As shown in FIG. 3, acasing or shell 21 has a plurality of slots 22, each of which has aperpendicularly extending portion 23. The slots are so disposed thatparallel portions of the different slots overlap each other. Thus, whena resin 24 is cast inside the shell the resin does not shrink from theshell during the curing process since the shell can contract with theresin. In

this manner, voids are prevented from forming between the resin and theshell in the same manner as withthecy:

lindrical sleeve 10 in which the slots 15 and 16 have overlappingportions.

The: shell: 2L maybe formed' by punching the slotsin, sheet of metalprior to: its being formed intotheboxdike shape. As previouslyexplained; aluminum foil. may be placed around: the 'shell; 21 toprevent the resin; from coming out through the slots when it is poured;into; the shell. The toil? maybe removed after the resin is cured. Fromthe foregoing; description, it is apparent that the present-inventionpermits a shelh orcontainer used as a IQOIdI for casting resin to.shrink with the resin. during: the curing. process, thereby avoidingseparation oftheresin and the container. Thisltstgucturef providesincreased mechanical strength and eliminates. or greatly reduces afsource-of"radiointerferencefifi the shell is of metal and fiorms partof a high voltage insulating device. Termir. nal bushings andotherelectrical components may be constructed in themanner-hereinilescltibed.V

Since. numerous changes may, be, made irr the. above describedconstruction, and difierent embodiments, of the invention may bemade'without the departing from the spiritv and; scope thereof; itisintended, thatall matter contained in the foregoingde'scription or shownin the acc'o m.

panying. drawings shall be interpreted as'illustrative and not in alimiting sense. 'l'claini astray-invention? V V 1: An electricalcomponent comprising a shell having a plurality of slots. therein, a.thermoset' epoxy resin body adhered to the inside of'the shell, saidshell being. composed of a-material havingi substantially the sameceflicient of expansion as the resin in said body, and said slotshavingparallel overlapping-portions to permit the shell to contract withthe resin.

2. An electrical component comprising a metal shell 1 having a pluralityof slots around; its 'periphe fy; a thermoset resin bodyjadhered to theinsideof; the shell, and said slotshaving overlapping portions to permitthe shell to contract during shrinkageof the resin.. I

1 metal shell having ai pluralityrofi slots; therein, athermo rqund. tsu ace a he 'moa st epoxy r s bo dh r d to h in ide; o he s el ,,and, sas o s h y n ov lapping portions to permit the shell; to contract withthe resin. v

5. An electrical; component comprising a box-like 'shell having aplurality of slots therein, a thermostat epoxy. resinlbody-adhfl zfidrtolthe, inside of the shell; said shellbeing composedjof a materialhaving substantially thersame coefiicient of expansionas'the resin, insaid body, and said slots having angularly disposed. port-ions.

6; An electrical component comprising abox-lilgetmetalshellzhavinga-plurality of slots therein, a thermoset epoxyresin; body!adhered to theinside of the shell, and; said:

slots; haying, parallel: oyerlapping' portions to; permit: the; shell tocontnactv'withthe resin;

' 7;: Anelectrical component comprising a box-like sheet set epoxy resinbodyadheredto the inside oi the shell, and: said: slots havingperpendicular and parallel. portionsv to permit the shell tocontract inat least two directions duringshrinka'ge of theresin.

References Gited in the file of this patent UNITE? SIATES' PATENTS r, 11837-3 8 Carroll V Nov. 24, 1914 1-,489 ,-369 Steinberger Apr. 8, 1924 1767 4241 Wirth June 24, 1930 2,809,228 Duttpn n l "700a 8; 19-57 romanBATENTS 1 079 048- France ee May 19 1954

